1. Field of the Invention
The present invention relates to an improved method and apparatus for measuring relatively long, linear distances. More particularly, the present invention relates to method and apparatus for determining the distance between two components of an injection molding machine that are capable of relative (linear) movement.
2. Description of the Related Art
In the course of operating an injection molding machine, it is necessary to have accurate data concerning the position of various machine components. For example, the positioning of the injection unit relative to the mold, the distance that the screw moves within the barrel of the injection unit during injection, the length of stroke for the ejector mechanism, and the distance between the stationary and moving platens during operation of the clamp mechanism.
Considering specifically the position of the moving platen, for example, there are several aspects of operation where precise control is important. As the moving platen changes position to open/close the mold, efficient operation requires position feedback to identify such things as: (a) the position where the mold portions touch, (b) a suitable point to change from rapid traverse to a slower speed just before mold touch, and (c) the position to be maintained when the clamp is at full tonnage. Consistent, accurate information for these positions is necessary to enable the control to minimize cycle times while safely producing acceptable parts. Typically, prior art arrangements use mechanical position transducers, such as limit switches and linear or rotary potentiometers that have contacting components subject to mechanical wear and decreasing accuracy over time.
Various non-contact sensing systems, such as those based on photo sensing or magnetic sensing technology, are known to provide accurate position feedback for relatively short distances. However, given the small operating range (typically less than one inch) for sensors of this type, they have not been useful in determining the position of various components in injection molding machines.
Accordingly, it is an object of the present invention to overcome the deficiencies in the prior art arrangements, and to provide an improved method and apparatus for accurate measurement of linear distances in an injection molding machine which avoids the use of components that are subject to mechanical wear and breakdown.